1. Field of the Invention
The present invention relates to a method of forming a thin film. More particularly, the present invention relates to a method of forming a silicon nitride layer using a deposition furnace with a multi-aperture air injection apparatus.
2. Description of the Related Art
In the formation of thin films on a wafer, many deposition processes are deployed. A prominent method of forming thin films is chemical vapor deposition (CVD). To form a thin film by chemical vapor deposition, reactive gases are passed into a heated furnace so that the gases can react chemically with the wafers to form a thin layer of deposition on the wafer surface. In general, the deposition furnace for performing chemical vapor deposition can be classified into horizontal deposition furnaces and vertical deposition furnaces. Since a vertical deposition furnace tends to occupy less area than a horizontal deposition furnace, the vertical deposition furnace gradually replaces all horizontal deposition furnaces.
When a vertical deposition furnace is used to deposit a silicon nitride layer on wafers, reactive gases including dichlorosilane (SiH2Cl2) and ammonia (NH3) are passed into the deposition furnace via gas injection pipes at the bottom of the deposition furnace. The gases react with the silicon wafers on a wafer boat set up inside the deposition furnace so that a layer of silicon nitride is deposited on each wafer. However, the method of passing reactive gases from the bottom of a vertical deposition furnace prevents gases, in particular, gases with a low reaction proportion, from reaching the top of the deposition furnace. Therefore, thickness of the silicon nitride layer on various wafers within the same wafer boat will show significant variations. In a micro electro-mechanical system, the reactant used for forming a silicon nitride layer including dichlorosilane and ammonia. In order to form a low stress silicon nitride film, the gas flow rate of dichlorosilane is larger than that of the ammonia. However, in the process of deposition, ammonia with a relatively small gas flow rate can not diffuse to the top of the deposition furnace. Therefore, the silicon nitride layer on various wafers within the same wafer boat will show low uniformity. Since only part of the wafers in the wafer board has a film stress low enough suitable for further processing, overall productivity of the deposition furnace is lowered.
Furthermore, in a micro electro-mechanical system, a silicon nitride layer having a thickness between 7000 Å to 10000 Å, compared with that of a conventional device, is required. Hence, the silicon nitride layer within a micro electro-mechanical system is more vulnerable to peeling than the silicon nitride layer of a conventional device if internal stress is high. For example, the micro integrated circuit device of an inkjet print head utilizes capillary effect to draw ink from an ink cartridge. When the silicon nitride layer is set up as part of an integrated circuit device on the chamber wall, the peeling of the silicon nitride layer from the chamber wall due to excessive stress will prevent the integrated circuit device from drawing ink. Consequently, the inkjet print head will fail to work. Hence, if a conventional vertical deposition furnace is used to fabricate the silicon nitride layer on wafers for forming the micro electro-mechanical system, the mass production of the deposition furnace will be very hard.